System and method for controlling quasi-resonant induction heating devices

ABSTRACT

A control circuit for an induction heating device comprises a D.C. power supply, referenced to a ground connection and configured to supply power to the induction heating device. A switching device is configured to be selectively activated to control the induction heating device. The switching device is connected on one end to the ground connection. A resonant load is disposed between the D.C. power supply and the switching device, the resonant load comprises a capacitor and an inductor connected in a parallel configuration. At least one rectifying device is disposed in series with the resonant load and the switching device. The switching device is configured to control current from the D.C. power supply through the resonant load.

TECHNOLOGICAL FIELD

The present disclosure relates to an induction cooktop and, morespecifically, to an induction cooktop assembly comprising a plurality ofcooking zones.

BACKGROUND

Induction cooktops are devices which exploit the phenomenon of inductionheating for food cooking purposes. The disclosure provides for a varietyof improved assemblies for induction cooktops that may improveperformance and/or economical manufacture. Such improvements may serveto improve the utilization of induction-based cooking technologies.Accordingly, the disclosure provides for assemblies, systems, andmethods for induction cooktops.

SUMMARY

In at least one aspect, a control circuit for an induction heatingdevice is disclosed. The control circuit comprises a D.C. power supply,referenced to a ground connection and configured to supply power to theinduction heating device. A switching device is configured to beselectively activated to control the induction heating device. Theswitching device is connected on one end to the ground connection. Aresonant load is disposed between the D.C. power supply and theswitching device, the resonant load comprises a capacitor and aninductor connected in a parallel configuration. At least one rectifyingdevice is disposed in series with the resonant load and the switchingdevice. The switching device is configured to control current from theD.C. power supply through the resonant load.

In at least another aspect, a method for controlling an inductionheating device is disclosed. The method comprises supplying current froma D.C. power supply into an input node of a resonant load and emittingthe current from an output node of the resonant load. The method furthercomprises directionally conducting the current in a unidirectional pathfrom the output node of the resonant load to a switching node downstreamalong the unidirectional path from the output node. The method furthercomprises controlling a current conducted through the resonant load witha switching device.

In at least another aspect, a control circuit for an induction heatingcoil is disclosed. The control circuit comprises a D.C. power supply.The D.C. power supply is referenced to a ground connection andconfigured to supply power to the induction heating device. The controlcircuit further comprises a switching device configured to beselectively activated supplying driving current to the induction heatingcoil. The switching device is connected on one end to the groundconnection. A resonant load is disposed between the D.C. power supplyand the switching device. The resonant load comprises a capacitor andthe induction heating coil connected in parallel. A rectifying device isdisposed in series between the D.C. Power supply and the switchingdevice.

These and other features, advantages, and objects of the present devicewill be further understood and appreciated by those skilled in the artupon studying the following specification, claims, and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A is a top, plan view of an induction cooktop assembly comprisinga plurality of induction coils;

FIG. 1B is a top, plan view of an induction cooktop assembly comprisinga matrix of induction coils;

FIG. 2 is a circuit diagram demonstrating a control circuit for asingle, non-under-clamped quasi-resonant inverter;

FIG. 3A demonstrates simulated results for a system response of thecontrol circuit shown in FIG. 2 over a time interval;

FIG. 3B demonstrates simulated results for a system response of thecontrol circuit shown in FIG. 2 over a time interval;

FIG. 4 is a circuit diagram demonstrating a control circuit for a matrixof non-under-clamped quasi-resonant inverters;

FIG. 5 is a circuit diagram modified from the circuit shown in FIG. 4demonstrating a current path within the matrix resulting from an omittedrectifying device;

FIG. 6 is a circuit diagram demonstrating a control circuit for anemitter switched array of quasi-resonant inverters comprising switchingdevices arranged in series;

FIG. 7 is a circuit diagram demonstrating a control circuit for anemitter switched array of non-under-clamped quasi-resonant inverterscomprising switching devices arranged in series;

FIG. 8 is a circuit diagram demonstrating a control circuit for anemitter switched array of non-under-clamped quasi-resonant inverterscomprising switching devices arranged in series; and

FIG. 9 is a block diagram of an induction system comprising a controllerconfigured to control one or more switching signals configured tocontrol one or more quasi-resonant inverters in accordance with thedisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of description herein the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof shall relate to the device as oriented in FIG. 1. However, it isto be understood that the device may assume various alternativeorientations and step sequences, except where expressly specified to thecontrary. It is also to be understood that the specific devices andprocesses illustrated in the attached drawings, and described in thefollowing specification are simply exemplary embodiments of theinventive concepts defined in the appended claims. Hence, specificdimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

Conventional induction cooktops may comprise a top surface made ofglass-ceramic material upon which cooking units are positioned(hereinafter “cooking utensils”). Induction cooktops operate bygenerating an electromagnetic field in a cooking region on the topsurface. The electromagnetic field is generated by inductors comprisingcoils of copper wire, which are driven by an oscillating current. Theelectromagnetic field has the main effect of inducing a parasiticcurrent inside a pan positioned in the cooking region. In order toefficiently heat in response to the electromagnetic field, the cookingutensils may be made of an electrically conductive ferromagneticmaterial. The parasitic current circulating in the cooking utensilproduces heat by Joule effect dissipation; such heat is generated onlywithin the cooking utensil and acts without directly heating thecooktop.

Induction cooktops have a better efficiency than conventional electricresistive element cooktops. For example, heating cookware via inductionprovides for a greater fraction of the absorbed electric power to beconverted into heat that heats the cooking utensil. In operation, thepresence of the cooking utensil on the cooktop causes the magnetic fluxto be directed into the pan itself resulting in power being transferredto the pan. The disclosure provides for assembly arrangements andmethods for improved manufacturing and performance of inductioncooktops. In particular, the disclosure provides for control circuitarrangements for cooktops comprising a plurality of induction coils.

Referring to FIGS. 1A and 1B, exemplary induction cooktop assemblies 10are shown. A first induction cooktop assembly 10 a may comprise aplurality of induction coils 14 forming cooking zones on a cookingsurface 16. A second induction cooktop assembly 10 b comprises a matrix12 or array of induction coils 14 distributed over the cooking surface16. In each of the embodiments 10 a, 10 b, and various similar orcombined configurations, the induction coils 14 may be in communicationwith a controller 18. The controller 18 may be configured to selectivelyactivate the induction coils 14 in response to an input to a userinterface 20. The controller 18 may correspond to a control systemconfigured to activate one or more cooking regions formed by theinduction coils 14 in response to an input or user selection.

As later discussed in detailed reference to various exemplaryembodiments, the induction coils 14 may be supplied current via one ormore control circuits in communication with the controller 18. Thecontrol circuits may comprise switching devices that may be configuredto generate a variable frequency/variable amplitude current to feed theinduction coils 14. The switching devices implemented in variousembodiments of the disclosure may comprise a variety of switchingtechnologies and configurations. For example, in some embodiments, theswitching devices may comprise one or more power semiconductor devices.The power semiconductor devices may comprise one or more transistors,thyristors, metal-oxide-semiconductor-field-effect-transistors(MOSFETs), power MOSFETs, insulated gate bipolar transistors (IGBTs),switch controlled rectifiers (SCRs), etc. Accordingly, the disclosuremay provide for the induction coils 14 to be driven by a variety ofcontrol circuits to heat a cooking utensil 22 (e.g. pans, pots, etc.).

In some embodiments, the induction coils 14 may be independentlyactivated by the controller 18. The activation of the induction coils 14may be in response to a user-defined heat setting received via the userinterface 20 in conjunction with a detection of a cooking utensil 22 onthe cooking surface 16. In response to the user-defined setting and thedetection of the cooking utensil 22, the controller 18 may activate theinduction coils 14 that are covered by the cooking utensil 22.Accordingly, the cooktop assembly 10 may provide for the cooking surface16 to be selectively energized providing for a plurality of flexiblecooking zones that may be referred to as a “cook anywhere”functionality.

The user interface 20 may correspond to a touch interface configured toperform heat control and selection induction coils 14 for a cookingoperation. The user interface 20 may comprise a plurality of sensorsconfigured to detect a presence of an object (e.g. a finger of anoperator) proximate thereto. The sensors of the user interface 20 maycorrespond to various forms of sensors. For example, the sensors of theuser interface 20 may correspond to capacitive, resistive, and/oroptical sensors. In some embodiments, the user interface 20 may furthercomprise a display 24 configured to communicate at least one function ofthe cooktop assembly 10. The display 24 may correspond to various formsof displays, for example, a light emitting diode (LED) display, a liquidcrystal display (LCD), etc. In some embodiments, the display 24 maycorrespond to a segmented display configured to depict one or morealpha-numeric characters to communicate a cooking function of thecooktop 10. The display 24 may further be operable to communicate one ormore error messages or status messages from the controller 18.

Referring now to FIG. 2, in some embodiments, a control circuit 30 offor the induction cooktop assembly 10 may be implemented using a novelconfiguration. For clarity, the control circuit 30 shown in FIG. 2 isreferred to as the first control circuit 30 a. The first control circuit30 a may be implemented as a variant of a quasi-resonant inverter. Thevariant of the quasi-resonant inverter shown in FIG. 2 is referred to asa Non-Under-Clamped, Quasi-Resonant (hereinafter referred to as NUC-QR)Inverter 32. The NUC-QR inverter 32 comprises a rectifying device 34connected in series with a resonant load 36 and a switching device 38.As shown in FIG. 2, the rectifying device 34 is interposed along thepath between a D.C. power supply 46, a resonant load 36, and a switchingdevice 38.

The rectifying device 34 may be implemented as a semiconductor diode.Semiconductor diodes may include, but are not limited to, junctiondiodes, Silicon Diodes, Silicon Carbide Diodes, Schottky diodes, etc. Insome embodiments, the control circuits and corresponding components maybe referred to using specific identifiers (e.g. first, second, third,etc.). The specific identifiers may be used for clarity to distinguishamong the exemplary embodiments of the control circuits 30 demonstratedin the figures. However, such designations shall not be consideredlimiting to the scope of the disclosed configurations provided herein.Accordingly, the control circuits 30 and underlying components may becombined or implemented in combination without departing from the spiritof the disclosure.

The resonant load 36 may be formed by an inductor 40 representing one ofthe induction coils 14 and a capacitor 42, connected in series with therectifying device 34. Though demonstrated with the rectifying device 34located downstream along a current path 44, in some embodiments, therectifying device 34 may be located upstream of the resonant load 36,between the resonant load 36 and a direct current (D.C.) power supply46. A representation of the rectifying device 34 positioned upstream ofthe resonant load 36 is shown in phantom lines. In operation, thefunction of the rectifying device 34 is to prevent any return current toa D.C. bus 48 when the resonant voltage (Va, Vc) is less than zero. TheD.C. power supply 46 may comprise a voltage rectifier 50, configured torectify a mains input voltage 52 into direct current and output the D.C.voltage to the D.C. bus 48 and a ground connection 54. Additionally, therectifier 50 may comprise a D.C. bus capacitor 56, which may beconfigured to smooth the voltage of the D.C. bus 48.

The arrangement of the rectifying device 34 arranged in series with theswitching device 38 (e.g. an IGBT), may be referred to as a reverseblocking configuration. In operation, the rectifying device 34 isconfigured to prevent return current traveling upstream opposite to thecurrent path 44 normally flowing from the D.C. bus 48 to the resonantload 36. Accordingly, a duration of a resonant phase of the NUC-QRinverter 32 is extended, leading to an improved regulation range.Additional benefits of the operation of the NUC-QR inverter 32 mayinclude decreased electromagnetic interference (EMI) and improvedoperating efficiency when compared to conventional inverter topologies.In this configuration, the controller 18 may be configured to controlthe switching device 38 via a control signal 58 to generate anelectromagnetic field to inductively heat the cooking utensil 22 over anincreased operating range.

FIGS. 3A and 3B demonstrate simulated results of the system response ofthe first control circuit 30 a. Referring to FIGS. 2, 3A, and 3B, thecomponent types utilized for the simulation shown in FIGS. 3A and 3B areas follows: switching device 38 (IGBT—APT25GF100BN), rectifying device34 (Diode—STTH3010D), capacitor 42 (270 nF), and inductor 40 (80uH witha series resistor of 40). As represented in FIG. 3A, the waveforms ofthe NUC-QR inverter 32 include gate voltage V_(ge) applied to the gateof the switching device 38, the current I_(cres) through the capacitor42, I_(SW) through the switching device 38, and the current I_(coil)through the inductor 40. The waveforms demonstrated in FIG. 3Bdemonstrate the voltage difference V_(ce) across the switching device 38between the cathode voltage V_(c) and the emitter voltage V_(e). FIG. 3Badditionally demonstrates the voltage difference V_(ae) across therectifying device 34 and the switching device 38 between the anodevoltage V_(a) and the emitter voltage V_(e) and the difference betweenthe V_(ce) and V_(ae). Finally, FIG. 3B demonstrates the power lossW_(SW) of the switching device 38.

The waveforms demonstrated in FIGS. 3A and 3B are demonstrated over aplurality of time intervals t₀, t₁, t₂, t₃, t₄, and t₅. From timeintervals t₀ to t₁, a gate voltage V_(g)e or command voltage of theswitching device 38 is high. Accordingly, both the switching device 38and the rectifying device 34 are ON and the currents I_(SW) through theswitching device 38 and the current I_(coil) through the inductor 40 arethe same. Additionally, the voltages V_(ce) and V_(ae) are approximatelyzero. The phase denoted from t₀ to t₁ is called a charging phase of theinductor 40. At the time t=t₁, the switching device 38 is turned OFF bythe controller 18. Following the charging phase, the free evolutionphase or resonant phase begins and persists until the time t=t₅.

In the resonant phase from times t₁ to t₂, the capacitor 42 and theinductor 40 begin to resonate, exchanging energy. At the time t=t₂, thevoltages V_(ce) and V_(ae) are maximum. At this stage, the voltagesV_(ce) and V_(ae) must not exceed a voltage limit or breakdown voltageof the switching device 38. From times t₂ to t₃, the free evolution ofthe resonant group continues, and a negative voltage across therectifying device 34 begins to grow. At the time t=t₃, the voltageV_(ae) becomes negative and the rectifying device 34 remains reversepolarized. The rectifying device 34 remains reverse polarized until thetime t=t₅, when V_(ae) becomes zero.

The operational phase associated with the proposed first control circuit30 a occurring from times t₃ to t₅ does not occur in conventionalconfigurations that have previously been implemented. Indeed, inconventional inverters, an anti-parallel diode is typically used tolimit the negative voltage difference V_(ce) to zero across theswitching device 38 between the cathode voltage V_(c) and the emittervoltage V_(e). In contrast, and according to the present disclosure, theNUC-QR inverter 32 of the first control circuit 30 a does not clamp thevoltage V_(ce), allowing the voltage V_(ae) to vary freely to negativevalues. During the phase from times t₃ to t₅, there is no current flowin the switching device 38 because the rectifying device 34 is reversepolarized.

The reverse polarization of the rectifying device 34 is caused by anegative voltage at the anode of the rectifying device 34 (i.e. nodeV_(a)). Between the time instants t₃ and t₅, the current passing throughthe inductor 40 (I_(coil)) is supplied by the capacitor 42. Therefore,there are no losses in the switching device 38, which results in animproved operating efficiency in comparison to conventional inverterarrangements. Finally, at the time t=t₅, the voltage across therectifying device 34 (V_(ae)−V_(ce)) crosses zero. At this time, theswitching device 38 begins to close the path for the current passingthrough the inductor 40 (I_(coil)).

The beneficial configuration of the first control circuit 30 a and theNUC-QR inverter 32 enables an increased timing range for the activationof the switching device 38 while maintaining soft-switching operation.For example, during the phase from times t₃ to t₅, the switching device38 may be controlled to turn ON (e.g. at t=t₄) without incurring inhard-switching losses. The soft-switching range is substantiallyextended because the commutation at high voltage levels V_(ce) acrossthe switching device 38 does not involve the discharge of the largeresonant capacitor as required by conventional systems. Instead, only arelatively small parasitic capacitance is associated with the switchingoperation of the switching device. The power loss associated with theparasitic capacitance is shown in FIG. 3B as a power loss 60 at theoutput of the switching device 38. Accordingly, the operation of thefirst control circuit 30 a comprising the NUC-QR inverter 32 providesfor improved efficiency by limiting loss associated with controlling theswitching device 38 and also extending the operating range the inverterwhile maintaining soft-switching operation.

Another important aspect of the present disclosure, particularly whenthe switching device is embodied as for instance an IGBT, is thewidening of the power delivery curve as a function of the IGBT ON time,with an increase in the maximum power being delivered to the inductioncoil 14 for a given maximum resonant voltage at the IGBT collector. Thisincrease in maximum power is due to the use of a larger fraction of theenergy stored in the capacitor 42 in the resonant load 36 during thephase t₃-t₅, where the V_(ae) is negative. In fact, in the conventionalquasi-resonant inverter, this phase is blocked by the anti-paralleldiode of the IGBT.

The particular arrangement of the first control circuit 30 a and theNUC-QR inverter 32 may be implemented in a variety of ways to providefor the improved operation of various devices for induction cooking andheating. The following discussion provides for similar novelconfigurations of control circuits 30 that may incorporate the operationof similar circuit configurations to achieve similar benefits to thosediscussed in reference to the first control circuit 30 a. Accordingly,the following exemplary embodiments of control circuits may beimplemented alone or in combination in various applications to providefor improved performance for induction heating and cooking.Additionally, common or similar elements of each of the control circuits30 may be referred to by like reference numerals for clarity.

Referring now to FIG. 4, a circuit diagram is shown demonstrating asecond control circuit 30 b for a matrix 62 of inverters 64. The matrix62 may comprise M rows 72 and N columns 70, where M=3 and N=2 in therepresentation are shown in FIG. 4. Similar to the first control circuit30 a, the second control circuit 30 b may implement a matrixconfiguration of the NUC-QR inverter 32. Accordingly, each of theinverters 64 forming the matrix 62 may be implemented as the NUC-QRinverter 32. As previously discussed, each of the NUC-QR inverters 32may comprise a rectifying device 34 arranged in series with the resonantload 36 and a switching device 38. More generally, each of therectifying devices 34 may be connected in series, upstream or downstreamto the resonant loads 36, along the resonant load current path 66. Asillustrated in the exemplary embodiment, each of the resonant loads 36may be formed by the inductor 40 and the capacitor 42 arranged inparallel and connected upstream along the current path 66 relative tothe rectifying device 34.

As shown in FIG. 4, the inductors 40 representing the induction coils 14are arranged in columns 70 and rows 72. Each of the columns 70 isconnected to the D.C. bus 48 via a column-switching device 74. Forclarity, the column switching devices 74 of each of the columns 70 maybe referred to as a first column switching device 74 a, a second columnswitching device 74 b, etc. Additionally, each of the rows 72 isconnected to a control input from the controller 18 via a row-switchingdevice 76. The row switching devices 76 of each of the rows 72 may bereferred to as a first row switching device 76 a, a second row switchingdevice 76 b, etc. The row-switching devices 76 are further in connectionwith the ground connection 54 of the voltage rectifier 50. In thisconfiguration, the controller 18 may selectively activate each of theinductors 40 to activate flexible heating zones on the surface 16 of thecooktop 10. Though the terms rows 72 and columns 70 are discussed inreference to each of the embodiments, it shall be understood that thearrangement of the rows 72 and columns 70 may be transposed withoutdeparting from the spirit of the disclosure.

The second control circuit 30 b may limit the specific combinations ofinductors 40 that can be energized by the controller 18 at a given time.In an exemplary embodiment, the induction coils 14 represented by theinductors 40 may be rated to supply an average power of up to 500 W anda peak power preferably comprised between 3 and 6 times the averagepower. Accordingly, each of the inductors 40 may operate with a maximumDuty Cycle equal to the ratio between the average power and the peakpower, wherein the ratio ranges from approximately 1:3 to 1:6. In thisway, the controller 18 may be configured to energize a limited number ofcoils at any given time. This operation inherently results in anincreased probability that an overlapping operating frequency range canbe achieved for multiple induction coils 14 operating simultaneously onone or more of the rows 72 or columns 70, resulting in the possibilityof the induction coils 14 to operate at the same identical frequency.

The presence of the rectifying device 34 provides for the second controlcircuit 30 b to prevent current from passing among the resonant loads36. Still referring to FIG. 4, a first column 70 a of the second controlcircuit 30 b comprises a first resonant load 36 a connected in serieswith a first rectifying device 34 a and a second resonant load 36 bconnected in series with a second rectifying device 34 b. Each of thefirst resonant load 36 a and the second resonant load 36 b are connectedto a first column 70 a. The first resonant load 36 a is furtherconnected to a first row 72 a, and the second resonant load 36 b isfurther connected to a second row 72 b.

The second control circuit 30 b further comprises a third resonant load36 c connected in series with a third rectifying device 34 c and afourth resonant load 36 d connected in series with a fourth rectifyingdevice 34 d. Each of the third resonant load 36 c and the fourthresonant load 36 d are connected to a second column 70 b. The thirdresonant load 36 c is further connected to the first row 72 a, and thesecond resonant load 36 b is further connected to the second row 72 b.As shown in FIG. 4, the rectifying devices 34 may prevent current frompassing among each of the resonant loads 36. Though specific numbers arereferenced to identify specific elements shown in the figures, suchreference numerals shall not be considered limited to the disclosure.

In FIG. 5 is illustrated an example of the second control circuit 30 bin which one of the rectifying devices 34 is omitted. As referred to inFIG. 5, the fourth rectifying device 34 d of the diagram shown in FIG.4, is omitted. During typical operation, as illustrated by thecorresponding dashed line in the Key for FIG. 5, the controller 18 mayactivate the first resonant load 36 a by activating each of a firstcolumn-switching device 74 a and a first row-switching device 76 a. Theswitching devices 38 are shown activated in response to a first signal58 a and a second signal 58 b transmitted from the controller 18.Accordingly, as shown, the current may flow from the D.C. bus 48,through the first column-switching device 74 a, through the firstresonant load 36 a, the first row switching device 76 a, and to theground connection 54.

The operation of the second resonant load 36 b is hereafter discussed inreference to FIG. 5, in which the fourth rectifying device 34 d isomitted. As illustrated in FIG. 5 by the corresponding dashed line inthe Key for fault operation, the omission of the fourth rectifyingdevice 34 d may result in the current passing through the secondresonant load 36 b and traveling along the second row 72 b toward thefourth resonant load 36 d. The current may further be conducted from thefourth resonant load 36 d upstream along the second column 70 b andthrough the third resonant load 36 c. As derivable from the abovedescription, the rectifying devices 34 may prevent current fromtraveling outward from one resonant load 36 and into another therebypreventing an unwanted working condition, wherein resonant loads 36 b,36 d, and 36 c are activated in addition to the only desired resonantload 36 a. Additionally, the utilization of the rectifying devices 34renders not necessary the use of switching devices with anti-paralleldiodes (e.g. reverse conducting IGBTs) such that simpler and lessexpensive switching devices may be utilized to construct the secondcontrol circuit 30 b.

Referring again to FIG. 4, each of the switching devices 38 of thesecond control circuit 30 b may be in communication with the controller18. In this configuration, the controller 18 may be operable tocoordinate the staggered activation of each resonant load 36 within thematrix 62. In such embodiments, the controller 18 may be configured tomonitor one or more electrical characteristics of each induction coil14. The controller 18 may monitor characteristics, such as current orvoltage supplied to each of the induction coils 14 via one or morefeedback inputs of the controller 18, which may correspond to analog ordigital inputs. The characteristics of each of the induction coils 14monitored by the controller 18 may include a complex impedance vs.frequency or the power vs. frequency curve. Based on the feedbackinformation from the induction coils 14, the controller 18 may computean activation sequence of predetermined duration T_(prog). Theactivation sequence may comprise a sequence consisting of N_(prog) timeslices of duration T_(s), wherein the control variables (period, dutycycle) of the switching devices are kept substantially constant. Asdiscussed herein, each of the control circuits 30 may comprise acontroller or control circuit configured to control the one or moreassociated switching devices. Further details regarding an exemplaryembodiment of the controller 18 are discussed in reference to FIG. 9.

The activation sequence of the controller 18 may correspond to a datastructure representing the switching frequency and duty cycle of theswitching devices 38 connected to each of the columns 70 and rows 72 inconnection with the resonant loads 36. For example, the controller 18may be configured to communicate an activation signal configured toselectively activate each of the column-switching devices 74 and therow-switching device 76 at each time slice T_(s) over the durationT_(prog) of the activation sequence. The time slice duration T_(s) maybe set equal to one semi-period of a frequency of the mains inputvoltage 52 or an integer number of semi-periods of the mains inputvoltage 52.

The activation sequence for the matrix 62 of the induction coils 14 maybe computed by the controller 18 with a plurality of constraints. Forexample, a first constraint may require that every time slice durationT_(s) for each row-switching device 76 be either idle (OFF) or operatingat a common frequency, equal for every resonant load 36 that is activein a particular semi-period of the frequency of the mains input voltage52, wherein the frequency may vary from one time slice T_(s) to another.A second constraint applied to the operation of the controller 18 mayrequire that each of the column-switching devices 74 be either idle(OFF) or closed (ON) for every time slice duration T_(s). A thirdconstraint may require that a Boolean matrix C_(d) defining the states(OFF/ON) of each inductor 40 in the matrix 62 must have a unitary rankfor every time slice duration T_(s). Finally, a fourth constraint mayrequire that the controller 18 controls the average power to eachresonant load 36 averaged over T_(prog) to be equal to a desiredsetpoint. Thanks to this control method, it is possible to energize in acontrolled manner the individual induction coils 14, 40 withoutincurring in unwanted cross-conduction.

As shown in FIG. 4, the matrix 62 is represented having M rows 72 and Ncolumns 70. The mains input voltage 52 may comprise a 2 phase or 3 phasedistribution system. Accordingly, the N columns 70 could be divided into2 or 3 groups in order to balance the power across the correspondingphases. For example, if N=8, a cooktop 10 is rated for a maximum powerof 7200 W at 230V could be split into two sub-matrices of N=4 columns 70each. Each matrix of the cooktop 10 may then be rated at a total powerof 3600 W, wherein each of the two matrices is connected to a differentphase of the mains input voltage 52.

The sub-matrices may be fed by one of the voltage rectifiers 50, whichmay be commonly connected to all of the columns 70 connected to the samephase of the mains input voltage 52. In this configuration, the commonvoltage rectifier 50 may provide for the voltage across each of the D.C.bus capacitors (e.g. D.C. bus capacitor 56) to be discharged to nearzero voltage at every zero crossing of the mains input voltage 52 whenpower is being delivered to at least one inductor 40 attached to thatparticular phase/sub-matrix. This operation may result in the beneficialeffect of allowing the possibility of the controller 18 to soft-startany of the inverters 64 at the next semi-cycle of the mains inputvoltage 52 because the voltage of the D.C. bus capacitor 56 isapproximately zero at this time.

Referring now to FIGS. 6-8, circuit diagrams are shown foremitter-switched arrays 82 of the induction coils 14, which arerepresented by inductors 40 of the resonant loads 36. For clarity, eachof the control circuits 30 demonstrated in FIGS. 6, 7, and 8 may bereferred to respectively as a third control circuit 30 c, a fourthcontrol circuit 30 d, and a fifth control circuit 30 e. The controlcircuits 30 c, 30 d, and 30 e may each be configured to control thecurrent supplied to a plurality of inverters 84 comprising the switchingdevices 38 arranged in series.

Each of the arrays 82 of the inverters 84 shown in FIGS. 6-8 maycomprise a plurality of the switching devices 38 connected in series.For clarity, the switching devices 38 may be referred to as a firstswitching device 86 a and a second switching device 86 b. The firstswitching device 86 a may be connected in series with each of theresonant loads 36. Additionally, each of the first switching devices 86a may be connected to a common, second switching device 86 b. The seriesconnection of the switching devices 86 a and 86 b may provide forimproved switching performance while minimizing cost. Each of thecontrol circuits 30 may be supplied power via the D.C. power supplies 46comprising the voltage rectifier 50. As previously discussed, thevoltage rectifier may be configured to rectify a mains input voltage 52into direct current and output the D.C. voltage to the D.C. bus 48 and aground connection 54. Further details of the specific configurations ofeach of the exemplary embodiments shown in FIGS. 6-8 are provided in thefollowing description. In general, the switching devices 86 a, 86 bdiscussed in reference to FIGS. 6-8 may be referred to as the switchingdevices 86.

Referring now to FIG. 6, the third control circuit 30 c may comprise thearray 82 of inverters 84 connected in parallel. The resonant loads 36may comprise the inductor 40 and the capacitor 42 connected in parallel.Each of the resonant loads 36 may be connected to the D.C. bus 48 andfurther connected in series with one of the first switching devices 86a. The first switching devices 86 a are connected to the secondswitching device 86 b via a common node 88. In this configuration, thecontroller 18 may be configured to drive the resonant loads 36 of thethird control circuit 30 c synchronously or in a time-multiplexed modeof operation.

In some embodiments, the first switching devices 86 a may correspond tohigh voltage devices with comparatively low switching speeds while thesecond switching device 86 b may correspond to a relatively low voltage,high switching speed device. In this configuration, the third controlcircuit 30 c may provide for a fast switching rate supported by thesecond switching device 86 b while controlling the high voltage of theresonant loads 36 with the first switching devices 86 a.

Referring now to FIG. 7, the fourth control circuit 30 d is shown. Thefourth control circuit 30 d may be similar to the third control circuit30 c and may further comprise the rectifying devices 34 arranged inseries with the resonant loads 36. As demonstrated in FIG. 7, each ofthe inverters 84 comprises the rectifying device 34 interposed betweenthe resonant loads 36 and the first switching devices 86 a. Therectifying devices 34 may prevent current passing among the firstswitching devices 86 a by blocking return currents in each of theinverters 84. Though demonstrated with the rectifying device 34 locateddownstream, in some embodiments, the rectifying device 34 may be locatedupstream of the resonant load 36, between the resonant load 36 and theD.C. bus 48. A representation of the rectifying device 34 positionedupstream of the resonant load 36 is shown in phantom lines.

Referring to FIGS. 6 and 7, the first switching devices 86 a may beimplemented as current controlled switching devices. Such devices mayinclude but are not limited to: bipolar junction transistors (BJTs),insulated-gate bipolar transistors (IGBTs), or other low outputimpedance devices. In exemplary embodiments, BJTs may be implemented tolimit cost and take advantage of the decreased switching speeds requiredfor operation of the first switching devices 86 a. The second switchingdevice 86 b may be implemented as a voltage controlled switching device.In an exemplary embodiment, the second switching device 86 b may beimplemented as a field-effect transistor (FET) or metal oxidesemiconductor FET (MOSFET).

In operation, the connection of the first switching devices 86 a and thesecond switching devices 86 b may provide for the controller 18 tocontrol the current supplied to the resonant loads 36 via a union ofactivation of one or more of the first switching devices 86 a incombination with the second switching device 86 b. In thisconfiguration, only one of each of the series connected pairs of theswitching devices 86 a, 86 b need to operate at the full switching speeddesired for operation of each of the resonant loads 36. For example, thefirst switching devices 86 a may be configured to operate at switchingspeeds significantly less than the second switching device 86 b. Suchoperation is demonstrated by the relative frequency of the first controlsignals 90 a supplied to first switching devices 86 a and the secondcontrol signals 90 b supplied to second switching device 86 b. In someembodiments, the controller 18 may control the first switching devices86 a to operate at a switching frequency less than 5 kHz, while thesecond switching device 86 b is controlled to operate at a frequencygreater than 5 kHz.

Additionally, the common connection of the first switching devices 86 ato the second device 86 b may provide for the control circuits 30 c, 30d, 30 e to supply a common switching signal to the second switchingdevice 86 b. The common frequency may be supplied by a pulse widthmodulator 92 operating at a constant frequency. The pulse widthmodulator 92 is demonstrated in FIG. 9 and may be implemented as adedicated circuit that may be controlled by the controller 18. In thisconfiguration, the individual activation of the first switching devices86 a may be actively controlled by the controller 18 at a relatively lowspeed. This configuration may provide for the controller 18 to havesignificantly simplified operational and computational processingrequirements, which, in turn, limit the cost of the controller 18 andrelated components of the control circuits 30 c, 30 d, 30 e.

Referring now to FIG. 8, the fifth control circuit 30 e is shown. Thefifth control circuit 30 e may similarly include the first switchingdevices 86 a connected to the common second switching device 86 b.However, the fifth control circuit 30 e may differ in that the firstswitching devices 86 a may be implemented as silicon controlledrectifiers (SCRs). The SCRs may provide for the beneficial function oflimiting current passing among the first switching devices 86 a upstreamof the second switching device 86 b. Accordingly, the fifth controlcircuit 30 e may not require the separate rectifying devices 34implemented in the fourth control circuit 30 d. Additionally, in thefifth control circuit 30 e, the second switching device 86 b may beimplemented as an IGBT rather than a MOSFET due to the high resonancevoltage of the inverters 84 passing through the first switching devices86 a.

Referring now to FIG. 9, a block diagram is shown demonstratinginduction system 100 comprising the controller 18. The controller 18 maybe configured to selectively activate the induction coils 14 representedby the resonant loads 36 in response to an input to the user interface20. The controller 18 may be implemented as a master controller of adistributed control system. Accordingly, the controller 18 may beconfigured to control one or more inverter controllers 102, pulse widthmodulators 92 or various other circuits configured to selectivelyactivate each of the induction coils 14. Accordingly, the controller 18may be configured to selectively activate one or more cooking regionsformed by the induction coils 14 in response to an input or userselection received by the user interface 20.

In general, the controller 18 may be configured to control one or moreswitching signals supplied to the switching devices 38 as discussed inreference to each of the control circuits 30. The controller 18 maycomprise a memory and may be configured to operate one or more controlschemes to selectively activate the induction coils 14 of the inductioncooktop 10.

The user interface 20 may correspond to a touch interface configured toperform heat control and receive a selection of the induction coils 14for a cooking operation. The user interface 20 may comprise a pluralityof sensors configured to detect a presence of an object (e.g. a fingerof an operator) proximate thereto. The sensors of the user interface 20may correspond to various forms of sensors. For example, the sensors ofthe user interface 20 may correspond to capacitive, resistive, and/oroptical sensors.

In some embodiments, the user interface 20 may further comprise adisplay 24 configured to communicate at least one function of thecooktop 10. The display 24 may correspond to various forms of displays,for example, a light emitting diode (LED) display, a liquid crystaldisplay (LCD), etc. In some embodiments, the display 24 may correspondto a segmented display configured to depict one or more alpha-numericcharacters to communicate a cooking function of the cooktop 10. Thedisplay 24 may further be operable to communicate one or more errormessages or status messages from the controller 18.

As demonstrated in FIG. 9, the control circuits 30 (e.g. the firstcontrol circuit 30 a, the second control circuit 30 b, etc.) aregenerally demonstrated in connection with the controller 18. According,the controller 18 may be configured to directly control the switchingdevices 38 or indirectly control the switching devices 38 in adistributed control configuration via the inverter controllers 102,modulators 92, or other similar control devices. The control circuits 30are in connection with the D.C. power supply 46. The D.C. power supply46 may comprise a voltage rectifier 50 configured to rectify a mainsinput voltage 52 into direct current and output the D.C. voltage to theD.C. bus 48 and a ground connection 54. Additionally, the rectifier 50may comprise a D.C. bus capacitor 56, which may be configured to smooththe voltage of the D.C. bus 48.

It will be understood by one having ordinary skill in the art thatconstruction of the described device and other components is not limitedto any specific material. Other exemplary embodiments of the devicedisclosed herein may be formed from a wide variety of materials unlessdescribed otherwise herein.

For purposes of this disclosure, the term “coupled” (in all of itsforms, couple, coupling, coupled, etc.) generally means the joining oftwo components (electrical or mechanical) directly or indirectly to oneanother. Such joining may be stationary in nature or movable in nature.Such joining may be achieved with the two components (electrical ormechanical) and any additional intermediate members being integrallyformed as a single unitary body with one another or with the twocomponents. Such joining may be permanent in nature or may be removableor releasable in nature unless otherwise stated.

It is also important to note that the construction and arrangement ofthe elements of the device as shown in the exemplary embodiments isillustrative only. Although only a few embodiments of the presentinnovations have been described in detail in this disclosure, thoseskilled in the art who review this disclosure will readily appreciatethat many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter recited. For example,elements shown as integrally formed may be constructed of multiple partsor elements shown as multiple parts may be integrally formed, theoperation of the interfaces may be reversed or otherwise varied, thelength or width of the structures and/or members or connector or otherelements of the system may be varied, the nature or number of adjustmentpositions provided between the elements may be varied. It should benoted that the elements and/or assemblies of the system may beconstructed from any of a wide variety of materials that providesufficient strength or durability, in any of a wide variety of colors,textures, and combinations. Accordingly, all such modifications areintended to be included within the scope of the present innovations.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions, and arrangement of the desired andother exemplary embodiments without departing from the spirit of thepresent innovations.

It will be understood that any described processes or steps withindescribed processes may be combined with other disclosed processes orsteps to form structures within the scope of the present device. Theexemplary structures and processes disclosed herein are for illustrativepurposes and are not to be construed as limiting.

It is also to be understood that variations and modifications can bemade on the aforementioned structures and methods without departing fromthe concepts of the present device, and further it is to be understoodthat such concepts are intended to be covered by the following claimsunless these claims by their language expressly state otherwise.

The above description is considered that of the illustrated embodimentsonly. Modifications of the device will occur to those skilled in the artand to those who make or use the device. Therefore, it is understoodthat the embodiments shown in the drawings and described above is merelyfor illustrative purposes and not intended to limit the scope of thedevice, which is defined by the following claims as interpretedaccording to the principles of patent law, including the Doctrine ofEquivalents.

What is claimed is:
 1. A control circuit for an induction heating devicecomprising: a D.C. power supply, referenced to a ground connection,configured to supply power to the induction heating device; a switchingdevice configured to be selectively activated to control the inductionheating device, the switching device being connected on one end to theground connection; a resonant load disposed between the D.C. powersupply and the switching device, the resonant load comprising acapacitor and an inductor connected in a parallel configuration; and atleast one rectifying device disposed in series with the resonant loadand the switching device, wherein the switching device is configured tocontrol current from the D.C. power supply through the resonant load. 2.The control circuit according to claim 1, wherein the rectifying deviceis embodied as a diode.
 3. The control circuit according to claim 1,wherein the rectifying device is arranged such that current is conducteddirectionally from the resonant load to the switching device.
 4. Thecontrol circuit according to claim 1, wherein the rectifying device isdisposed between the resonant load and the switching device.
 5. Thecontrol circuit according to claim 4, wherein the rectifying device is adiode connected to the resonant load on an anode side and to theswitching device on a cathode side.
 6. The control circuit according toclaim 5, wherein the diode prevents the reverse current passing from theswitching device to the resonant load and the power supply in responseto a voltage at the anode being less than zero.
 7. The control circuitaccording to claim 1, wherein the rectifying device is disposed betweenthe D.C. power supply and the resonant load.
 8. The control circuitaccording to claim 7, wherein the rectifying device is a diode connectedto the D.C. power supply on an anode side and to the resonant load on acathode side.
 9. The control circuit according to claim 8, wherein thediode prevents the reverse current passing from the resonant load to thepower supply in response to a voltage at the cathode side being greaterthan a D.C. power supply voltage.
 10. The control circuit according toclaim 1, wherein the switching device and the rectifying device arecombined forming a reverse-blocking switching device.
 11. The controlcircuit according to claim 10, wherein the reverse-blocking switchingdevice is a reverse-blocking IGBT.
 12. The control circuit according toclaim 1, wherein the rectifying device is configured to blockreverse-current returning through the switching device into the resonantload and the D.C. power supply.
 13. A method for controlling aninduction heating device comprising: supplying current from a D.C. powersupply into an input node of a resonant load; emitting the current froman output node of the resonant load; directionally conducting thecurrent in a unidirectional path from the output node of the resonantload to a switching node downstream along the unidirectional path fromthe output node; and controlling a current conducted through theresonant load with a switching device.
 14. The method according to claim13 wherein the resonant load comprises a capacitor and an inductorconnected in parallel between the input node and the output node. 15.The method according to claim 13, wherein the current is supplied fromthe power supply to the resonant load substantially without conductiveinterruption.
 16. The method according to claim 13, further comprising:blocking a reverse-current opposite the unidirectional path from passingthrough the switching device, the resonant load and the D.C. powersupply.
 17. The method according to claim 16, wherein the switchingdevice is activated from the instant when the current flowing in theresonant load is zero until the voltage across the resonant load returnsto less than the voltage of the D.C. power supply.
 18. The methodaccording to claim 17, wherein the diode prevents a reverse current frompassing from a switching device at the switching node to the D.C. bus inresponse to the voltage at the output node or the switching node beingless than zero.
 19. A control circuit for an induction heating coilcomprising: a D.C. power supply, referenced to a ground connection,configured to supply power to the induction heating device; a switchingdevice configured to be selectively activated supplying driving currentto the induction heating coil, the switching device being connected onone end to the ground connection; a resonant load disposed between theD.C. power supply and the switching device, the resonant load comprisinga capacitor and the induction heating coil connected in parallel; and arectifying device disposed in series between the D.C. power supply andthe switching device.
 20. The control circuit according to claim 19,wherein the rectifying device is embodied as a diode positioned either:in series between the D.C. power supply and the resonant load; or inseries between the resonant load and the switching device.